A device of this kind is known from EP 0 300 262 B1 (U.S. Pat. No. 4,995,253). For producing a bar of high quality in a bar or tube press it has been recognized as necessary for the bar to be withdrawn from the die of the bar or tube press with a defined withdrawal force. In this manner it can be ensured that the bar formation in the die will not be negatively influenced. To ensure a high quality of the bar a withdrawal device has been described with which the end of the bar to be withdrawn is affixed to a carriage movable on a rail. The carriage is connected with drive means so that a tension force can be applied to the bar. For optimal finishing results, the tension force is a combination of different components, whereby especially one component is determined as a function of the pressed-out bar length.
In this manner the cross sectional tolerances of the bar withdrawn from the bar press can be reduced so that the bar has the desired quality. This system is based upon the consideration that the bar, with increasing spacing from the die, as a result of the cooling, can be loaded with increasing tension without producing a cross sectional change.
It has been found problematical in this context that different forces are applied to the withdrawal carriage over the withdrawal path so that it is very difficult to ensure a defined withdrawal force, for example conforming to a predetermined function, which can be applied to the bar end.
The carriage or puller truck has a mass and is driven by a motor. The speed and torque of the motor can be predetermined by a control. The motor force applied to the carriage, which is converted into the withdrawal force applied to the extrusion pressed profile or strand, will depend upon the friction force of the mounting of the carriage as well as the initial forces which are dependent upon the actual acceleration of the carriage. Only in a static case will the motor force correspond to the withdrawal force, because in that case the speed and the friction are zero. When the profile or strand moves, the friction force and inertial forces on the carriage are not constant and the friction coefficient will depend upon environmental conditions (temperature, contamination of the path). As a consequence it becomes very difficult to apply a defined predetermined withdrawal force to the bar to be withdrawn. If the carriage is accelerated at the beginning of the withdrawal process the force equilibrium is destroyed as a consequence of the inertial forces which arise so that there are special difficulties in maintaining a defined withdrawal force. With light and sensitive profiles or strands this can give rise to a problem with respect to the quality of the product or to an increase in the waste level in production.